The effect of halothane an inhalation anesthetic, on the steady state kinetics of Ca 2ion uptake by the cardiac sarcoplasmic reticulum has been examined. Halothane has a biphasic effect on Ca 2ion uptake; it can either activate or inhibit Ca 2ion uptake. We have examined the relationship of halothane to the substracts ATP and Ca 2ion, and the cofactor Mg 2ion with respect to activation or inhibition of Ca 2ion uptake.